1. Field of the Invention
The present invention is directed to a pressure pulse generator, and in particular to a pressure pulse generator of the type having an electrodynamically operated coil and membrane as a pressure pulse source.
2. Description of the Prior Art
Pressure pulse generators can be used for a variety of purposes, for example in medicine to non-invasively disintegrate calculi situated in the body of a patient, or to non-invasively treat pathological tissue conditions. Positive (greater than atmospheric) pressure pulses are employed in the former case, and negative or rarefaction (less than atmospheric) pressure pulses are preferably employed in the latter case. Such pressure pulse generators can also be used for testing materials by charging specimens with pressure pulses.
A pressure pulse generator is always acoustically coupled in a suitable manner to the subject being acoustically irradiated, so that the pressure pulses generated in the acoustic propagation medium in the pressure pulse generator can be introduced into the subject. The pressure pulse generator and the subject must thus be aligned relative to each other so that the region of the subject to be acoustically irradiated is situated in the propagation path of the pressure pulses. If the pressure pulse generator generates focused shockwaves, it must also be assured that the region of the subject to be acoustically irradiated is situated in the focal region of the pressure pulses.
A pressure pulse generator of the type generally described above is disclosed in U.S. Pat. No. 4,674,505. This type of pressure pulse generator is known as an electromagnetic or electrodynamic shockwave generator, and produces positive pressure pulses using a coil arrangement. When the coil is charged with a high-voltage pulse, a magnetic field is generated extremely quickly. This magnetic field induces a current in a membrane disposed opposite the coil arrangement. The membrane contains conductive material, and current is thus caused to flow in the membrane in a direction opposite to the current flowing through the coil. A magnetic field opposite in direction to the magnetic field associated with the coil arrangement is thus generated in the membrane. This causes the membrane to be rapidly repelled from the coil arrangement as a consequence of the magnetic repulsion forces. A pressure pulse is thereby introduced into an acoustic propagation medium adjacent the membrane, and gradually intensifies to form a shockwave along its propagation path.
Such shockwave generators have a relatively low efficiency, so that a large part of the electrical input energy is converted into heat. This results in the pressure pulse source gradually heating during use. Because elevated operating temperatures of the pressure pulse source can result in a premature failure thereof, particularly due to a failure of the membrane as a result of high mechanical stresses, it is provided in a pressure pulse generator disclosed in European Application 0 265 741 that the acoustic propagation medium be conducted through a circulating cooling system during operation. This makes it possible to eliminate the heat emitted by the pressure pulse source which is contained in the acoustic propagation medium. In the case of this known pressure pulse generator, however, the cooling effect is not sufficient under all circumstances. In this known pressure pulse generator, "heat pockets" can form for which a sufficient heat elimination is not guaranteed.
Another problem associated with pressure pulse sources of the type described above is that the membrane must be returned to its initial position after the production of a pressure pulse. Only in this way is it insured that the membrane will have a defined initial position, in which it is pressed against the coil arrangement, before a shockwave is generated. This is a prerequisite for insuring that the acoustic characteristics of successively generated shockwaves coincide with adequate precision. In a pressure pulse generator disclosed in European Application 0 188 750, the return of the membrane to its initial position is accomplished by charging that side of the membrane facing away from the acoustic propagation medium with negative pressure. Although this insures a reliable return of the membrane to its initial position, a substantial design outlay is required, and a negative pressure source must be provided.
A pressure pulse generator is disclosed in German OS 33 12 014 wherein the membrane is returned to its initial position by a pressurized propagation medium.